In a fossil fuel power plant, coal or natural gas are burned in a combuster. The combustion gas also known as flue gas, heats water to form steam which turns a turbine generator to produce electric power. The flue gas steam is passed through an air preheater such as a rotating wheel heat exchanger, that transfers heat from the flue gas to an incoming air stream that thereafter flows to the combuster.
The flue gas contains the products of combustion, including solid particulates, and gases such as nitrogen and carbon dioxide. In addition, various pollutants gases may be contained in the flue gas stream, most notably, nitrogen oxides, more commonly known as NO.sub.x. Such NO.sub.x emissions, must be reduced to the maximum extent possible. To remove NO.sub.x, a compound such as ammonia is typically injected into the flue gas stream. The ammonia reacts with the NO.sub.x to form nitrogen and water, reducing the NO.sub.x content of the flue gas. The reaction of ammonia and NO.sub.x may be performed at high temperature without a catalyst, a process known as "selective non-catalytic reduction" (SNCR). It may also be performed at lower temperature in the presence of a catalyst, a process termed "selective catalytic reduction" (SCR).
In U.S. Pat. No. 4,060,273, there is described the use of heat exchanger elements coated with a catalyst. The heat exchanger elements are located in flue gas such that NO.sub.x conversion is accomplished. In the disclosed device, the catalyst is located on the circulating regenerative heat exchanger which rotates between the hot gas stream and the cold gas stream. This is necessary to prevent the catalyst from degrading in the hot gas stream. Consequently, the temperature of the catalyst continually changes as the heat exchanger elements rotate from the hot gas stream to the cold gas stream.
In U.S. Pat. Nos. 5,145,652 and 4,940,567, another apparatus for removing nitrogen from flue gas is discussed which has catalyst disposed on heat exchanger elements. These patents again utilize regenerative type heat exchanger which moves from a hot gas stream to a cold gas stream, such that the catalyst is subject to temperature cycling. As an alternative, occasionally the catalyst is provided in the form of a fixed catalyst bed through which the flue gas must pass. However, due to the physical temperature limitations of the catalyst, use of a stationary catalyst bed requires that the flue gas be cooled, which correspondingly results in a reduction in the activity of the catalyst, with a consequent reduction in the ability to remove NO.sub.x from the flue gas stream. Such fixed beds also increase significantly the capital cost of the system, require periodic extensive shutdowns for catalyst cleansing and add significant pressure drop to the gas flow system. Thus, such systems are not favored in the industry.
Another problem with existing systems, beyond the extreme catalyst cycling is the natural variations in temperature of the flue gas stream. This can vary depending on the incoming gas temperature, power output requirements, type and quantity of fuel, etc. Thus, conversion efficiency, which depends on temperature, can vary on a daily basis.